In drilling an oil well, the hole will drift somewhat from the true vertical reference normally intended for the path of the well. As it drifts, the deviation from the vertical causes the drill string to bear against the sidewall. When this occurs, the extra drag wears on the drill string. It runs the risk of differential pressure sticking. Moreover, drifting holes sometimes form what is called a "key seat." A key seat is a round hole which is originally cut by operation of the drill bit which has been cut at one side by rubbing from the drill string to form an opening which resembles, in cross section, a keyhole.
Most drill strings are drilled with externally upset drill pipe, the external upsets occasionally hanging in the key seats. The present invention is an enhanced version of apparatus which, placed in a drill string, enlarges the hole where rubbing occurs between the drill string and a key seat. It includes a type of drill collar with an external helical thread cut on it with the edges of the thread shaped to cut a portion of the key seat away, thereby enlarging the key seat area. This enables the broach of the present invention to reshape the drilled hole. As the drill string is rotated, the broach cuts into constrictions impeding rotation and penetration of the drill string. The broach incorporates a helical thread formed on it with a slightly raised, hard metal reinforced crown on the thread which cuts into the hole, thereby enlarging it. To the extent the hole is straight, the broach does not cut. Rather, it cuts only where the drill string must pass through a constriction or drags against the well bore.